1. Field of the Invention
The present invention relates to a measuring method using a spectrum analyzer, and more particularly, to a method for setting an input level (reference level) of a spectrum analyzer, a method for measuring a carrier (or carrier wave) power of an input signal and a method for measuring a spurious power or a leakage power to adjacent channels.
2. Description of the Related Art
In a conventional measuring method using a spectrum analyzer, it is required to set an input level, namely a reference level of the spectrum analyzer so that a proper measurement can be performed. For this setting, the spectrum analyzer is maintained in a sweep measurement state and a variable attenuator at the input stage is adjusted observing the measured level displayed on a display to maintain a secondary harmonic level in unchanged state. That is, by this adjustment operation, a secondary distortion generated by first stage frequency converting means can be neglected and an input signal is maintained, in the first stage frequency converting means, in undistorted state.
In the case of measuring a spurious power or a leakage power to adjacent channels, a carrier power of an input signal is first measured. In the measurement of a carrier power, a carrier power is obtained by a so-called sample detector wherein a measuring central frequency of a spectrum analyzer is matched with the carrier frequency of the input signal and an indication on a frequency axis on the display screen determined by a set sweep speed is sampled to calculate a power from each sample value at that frequency.
After that, a spurious signal is searched, in the state that the spectrum analyzer is maintained in a frequency sweep mode, to measure a spurious power. Alternatively, a leakage power to adjacent channels defined is measured in accordance with an input signal in the frequency sweep mode.
This measured spurious power or leakage power to adjacent channels is converted to a relative value to the carrier power obtained as described above. The relative value of the spurious power or the leakage power to adjacent channels is checked whether or not this value is less than a predetermined value to judge goodness or badness of the measurement result.
As described above, in the prior art, an operator observes a secondary distortion power by setting a reference level of a spectrum analyzer to, for example, a minimum value and thereafter, the reference level is manually changed until its secondary distortion power becomes a constant value (no changing state) and the reference level the secondary distortion power of which is constant is used as a proper reference level, namely an input level. Therefore, there has been a problem that it takes a long time period and a large workload for setting a proper reference level.
In addition, in the prior art, as described above, a carrier power is measured by a sample detector. In the sample detector, since the sample values have a relatively wide dispersion, a plurality of sweep operations are performed to obtain an average value of the sample values and the average value is used as a measured value of a carrier power.
Moreover, since, in a sample detector, a sample data displayed on a display is used as mentioned before, i.e., since an output of a logarithmic amplifier is detected by a detector and a sample data of the detected output is used, a carrier power is obtained by averaging a plurality of sweep operation results from the stand point of avoiding the influence of errors of the logarithmic amplifier and the detector. As described above, in the prior art, since a plurality of sweep operations are necessary, there is a drawback that it takes a long time period and a large workload in the measurement of a carrier power.
It is an object of the present invention to provide a measuring method using a spectrum analyzer wherein a reference level of the spectrum analyzer can automatically be set.
It is another object of the present invention to provide a measuring method using a spectrum analyzer wherein a carrier power of an input signal can accurately be measured in a short time period without requiring a plurality of sweep operations.
According to the present invention, there is branched an output of frequency converting means for frequency selection in the spectrum analyzer, the branched output is converted into a digital data, a check is made to see if AD converting means for converting the branched output into the digital data has overflowed, and if the AD converting means has overflowed, first stage level adjusting means of the spectrum analyzer is adjusted to prevent the overflow from occurring. In such a manner, a reference level is automatically set.
Further, after the adjustment of the level adjusting means, an output level of the frequency converting means for frequency selection is adjusted in a smaller unit than an adjusting unit of the first stage level adjusting means such that a peak value of the digital data falls within a predetermined range in the full scale of the AD converting means, namely, such that an input level of the AD converting means is always within the proper input level range of the AD converting means.
In a measurement of a carrier power, a measuring central frequency of the spectrum analyzer selected by the frequency converting means for frequency selection is set to a carrier frequency of an input signal and a sweep of measuring frequencies is stopped, and a carrier power of the input signal is calculated from the digital data.
Alternatively, the digital data is demodulated and based on the demodulated data, a carrier power is calculated for a sample of the digital data predetermined with respect to the input signal.
Measuring frequencies are swept in the state that the measuring central frequency is set to the carrier frequency of the input signal to measure a spurious power or a leakage power to adjacent channels.
A standardized template in accordance with the input signal is displayed based on the carrier power, and a spectrum measured by sweeping the measuring frequencies is displayed overlapping with the template.